The present invention relates to a polyester film which, when made into a magnetic tape, shows excellent electromagnetic properties, running properties and abrasion resistance. More particularly it relates to a base film for high-density magnetic recording media, which scarcely has scuffing and generates less abrasion dust during running of the tape and also enables easy cut of the magnetic tape made by using the film.
Polyester films have excellent physical and chemical properties and are widely used in the industries. Especially biaxially stretched polyethylene terephthalate films are superior to other types of film in flatness, mechanical strength and dimensional stability and now are essential as base for magnetic recording media.
On the other hand, improvements on magnetic recording media have been made rapidly in recent years, and this naturally has resulted in a severe requirement for quality of base film. For instance, base film is required to have a surface with a high degree of flatness for use in a magnetic tape, such as videotape, where high-density recording is made. However, as is well known, the higher the flatness of film surface, the greater becomes the friction between the film and the parts contacted therewith. This increases the risk of scuffing of the film and generation of abrasion dust to give rise to various troubles.
For example, if abrasion dust is generated at a production stage prior to the magnetic layer coating step, there may take place skips of coating of the magnetic layer or deposition of abrasion dust on the magnetic layer surface to cause increase of frequency of dropout. Also, if abrasion dust is generated in the course of calendering which is conducted for the purpose of flattening the magnetic layer surface, such abrasion dust will deposit as white powder on the calender roll surfaces to cause roughening of the magnetic layer suface and a deterioration in electromagnetic properties of the product. Further, if abrasion resistance of the film is poor, abrasion dust might be generated in the tape running system within the tape deck even after made into a product such as magnetic tape to cause lowering of electromagnetic properties or more frequent occurrence of dropout.
Recently, request is rising for the improvement of resistance to scuffing in addition to abrasion resistance of base film. Such a request is based on the fact that a tape may suffer scuffing by recording elements contact with the tape in the course of tape dubbing or other steps, or the fact that generated abrasion dust causes scuffing to the film. Such scuff marks are not only undesirable from viewpoint of appearance of appearance but also tend to cause contamination of working steps or increase of dropout.
It is known that abration resistance and scuff resistance can be improved by incorporating inactive fine particles in polyester film to roughen the film surface to a proper degree. Such moderate surface roughening is not only helpful for improving abrasion resistance and scuff resistance but also contributes to improvement of film workability or running properties of magnetic tape using the film, but this approach can not meet with recent strict requirements in quality. It is necessary for well satisfying such quality requirements to increase the amount of fine particles blended or increase the particle size, but this approach tends to raise the average surface roughness to an excessive degree or to increase the number of coarse surface protuberances due to the presence of coarse particles, resulting in a deterioration in electromagnetic conversion characteristics of magnetic tape or an increase in the frequency of dropout. It is quite difficult to satisfy these antinomic requirements at the same time. The only known effective method is so-called back coating, where an appropriate coating is formed on the back side of the film (the side opposite from the magnetic layer). This method, however, greatly increases the manufacturing cost and also involves many problems relating to film properties, so that this method has not been deemed practically suitable.
Another important property required of the base film for magnetic recording materials to possess is easiness to slit. If the base film is poor in cutting quality, when magnetic tape is slitted into the pieces of a given width, burrs are made at the slit end and, in some cases, the tape material may come off as powders from the slit end. When this phenomenon becomes more remarkable, fall-off of material in the form of powder from the magnetic layer may be caused, and such fallen-off powder may become a cause of dropout.
As a means for improving slitting quality, a method is known in which crystallinity of film is increased. However, an increase of crystallinity leads to deterioration of abrasion resistance, so that it is practically unsuitable to raise the crystallinity to excess. Currently, therefore, designing of base film is made by sacrificing the slitting quality to some extent.
Thus, it would be of great industrial significance if techniques were found by which it is possible to effectively improve, with simple means, abrasion resistance, scuff resistance and slitting quality (easiness to cut) of base film for magnetic recording media.
In view of the above, the present inventors have made strenuous studies on polyester films for magnetic recording media and, as a result, found that a polyester film containing certain specified fine particles has excellent abrasion resistance, scuff resistance and cutting quality (easiness to cut). The present invention was accomplished on the basis of such finding.